Circuit arrangements of this type are used, for example, in chip cards, particularly chip cards with contacts. A plurality of voltage ranges for the externally applied voltage are prescribed by ISO 7816-3 for such chip cards. Permitted voltage ranges are accordingly 5.0 volts ±10%, 3.0 volts ±10% and 1.8 volts ±10%. Within the chip, the voltage regulator for generating a regulated operating voltage ensures a constant operating voltage of typically 1.5 volts which is suitable for the present technology. Despite the voltage regulator, load fluctuations or fluctuations in the external voltage often make it impossible to keep the operating voltage in the range of 1.5 volts ±10% under all circumstances.
In this case, particular importance is attached to hacker attacks which deliberately manipulate the voltage which is supplied to a chip card in order to disrupt data processing within the chip card, which may result in it being possible to read out data which are intended to be kept secret or to detect internal processing operations which are veiled during normal operation. In order to prevent hacker attacks of this type, provision is made of the voltage monitoring unit which monitors the regulated operating voltage and generates an alarm signal when the prescribed permissible voltage interval is left, said alarm signal preferably resulting in the system being reset. Suitably setting the permissible voltage interval is problematic in this case. On the one hand, this interval must be so small that malfunctions can be guaranteed not to occur, but, on the other hand, the interval must be so large that internal voltage fluctuations during normal operation do not trigger a reset since the system does not operate correctly otherwise.
The permissible voltage interval has hitherto been selected to be so large that no alarm is triggered during normal operation. This led to increased design complexity since the circuit must be guaranteed to operate reliably in this large voltage interval, which is all the more problematic, the lower the operating voltage. Another known measure is to keep load fluctuations as low as possible using a complicated circuit design so that the prescribed voltage limits of the voltage interval do not lead to the alarm in the case of normal load changes. The disadvantage of the two known measures is the increased complexity of the circuit design and the associated increased area requirement of the circuit arrangement.